1. Field
The following description relates to electronic packaging, and more particularly, to a molded surface-mount device (SMD) light-emitting diode (LED) display module with reduced color shift that can be used in various lighting applications.
2. Description of Related Art
LED packages are assemblies that house LED chips. There are many types of LED packages in use today, each with particular applications and advantages. The functions of LED packages include protecting the LED chips and welded lead wires from direct contact with the environmen, securing and encapsulating the chip with epoxy or silicone resin, and allowing for versatility and stanardization in LED lighting applications.
The SMD LED display module is an LED display module that uses surface-mount technology (SMT) to mount LED packages onto a printed circuit boards (PCB) surface via welding. SMT is a method for producing electronic circuits in which the components are mounted or placed directly onto PCBs. An electronic device so made is called a surface-mount device (SMD).
The surface-mount LED device is designed to function on its own or to be plugged into a compatible unit. There are many types of SMD LED modules that are distinguishable according to the physical dimensions of the LED chip.
After being welded onto circuit boards, SMD LED modules are widely used in backlighting for computer monitor or TV screes, or in LED lamps for home illumination, advertising, automobile interior lighting and numerous other lighting applications. The brightness may vary depending on the driving current. The output light and consumed power depend on the LED chip.
SMD LEDs are often used in LED light strips and can be installed in a variety of places because of their excellent versatility. The packaging material is typically made of ceramics or plastics, though many plastics cannot withstand the high LED junction temperature.
The SMD LED display module generally comprises a driver PCB carrying a large number of surface-mount LED devices arranged in rows and columns. Each SMD LED device may carry a vertically oriented, linear array of red, green and blue LEDs defining a pixel. The SMD LED devices are electrically connected to pads on the PCB connected to respond to appropriate electrical signal processing and driver circuitry.
Different types of SMD LEDs according to the package structure are available. The first type of the SMD LED is a top-emitting LED (“Top LED”) that comprises a black sidewall defining a cavity, LEDs placed on a lead frame, and a transparent resin encapsulation molded around the LED chips. The top-emitting LED or Top LED is typically used for electronic equipment such as dashboard and signal LED board. It is widely used for indoor and outdoor decorative light.
In the top-emitting LED, light generated by each LED is emitted upward and sideward. However, because of the black sidewall formed around the LED chips, the light emitted sideward is blocked so that the light is emitted only upward. One issues of this technology is that the upper surface of the Top LED is generally uneven due to the resin filling and curing process. The structure of the top-emitting LED will be described in more detail in the detailed description section.
The second type of the SMD LED is a side and top-emitting LED (“Chip LED”) comprising LED chips placed on a lead frame, bonding wires connecting the LED chips to the lead frame, and a transparent resin encapsulation molded around the LED chips.
The use of the Chip LED is similar to that of the Top LED. However, unlike the Top LED, the Chip LED emits light both in the top and side directions because the LED chips are not surrounded by the black sidewall. Further, unlike the Top LED, the top surface of the Chip LED is generally flat. The structure of the Chip LED will be further described in the detailed description section below.
When two or more Top LEDs are put together on a PCB to form a SMD LED display module, the resulting light pattern exhibits a certain level of color shift over viewing angles due to the uneven top surface and variation of the uneven top surface between Top LEDs. Because the light generated by each LED bounces within the cavity defined by the black sidewall before the light is emitted in the vertical direction, all red, green and blue color radiations closely follow Lambertian light pattern. Accordingly, the color shift over the viewing angle variations in the Top LED is less than that in the Chip LED.
However, for Chip LEDs, the light radiated from all sides of the Chip LED causes a more severe color shift over the viewing angle variations than the Top LED because of the irregularity of resin cutting and the sharp angle of the encapsulation in a square shape, which causes light leakage and results in noisy images.
When a color shift occurs in the SMD LED display module, the image becomes blurred, and the contrast and uniformity of the image are deteriorated. Therefore, there is a need for an improved packaging method and structure that can reduce the occurrence of the color shift over the viewing angles in the SMD LED display module.